Change some Datum to void * for opaque pass-through pointer

Change some Datum to void * for opaque pass-through pointer

Here, Datum was used to pass around an opaque pointer between a group
of functions.  But one might as well use void * for that; the use of
Datum doesn't achieve anything here and is just distracting.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/1c5d23cb-288b-4154-b1cd-191fe2301707%40eisentraut.org